Li-ion batteries (LIBs) remain the most prominent rechargeable, electrochemical energy storage technology [1], with tremendous importance for portable electronics as well as for the rapidly growing sector of environmentally-benign, electrical mobility [2]. A conceptually identical technology, Na-ion batteries (SIBs), is also emerging as a viable alternative due to much greater natural abundance and more even distribution of Na as compared to Li. The particularly strong appeals of commercialized LIBs are their long operation life span, over hundreds to thousands charge/discharge cycles, and superior and broadly tunable balance between the energy density and the power density [3]. This implies, inter alia, that in the search for alternative Li-ion anode materials not only reversible theoretical charge-storage capacities must be higher than that of Graphite (372 mAh g−1), but also satisfactory retention of capacity on a long-term and under fast charge/discharge cycling (high current densities) must be obtained. For instance, the transition from commercial graphite anodes to most intensely studied alternatives such as Si, Ge, Sn and some metal oxides, with 2-10 times higher theoretical capacities (with 3579 mAh g−1 for Si being the highest) [4] is primarily hampered by the structural instabilities caused by drastic volumetric changes up to 150-300% upon full lithiation to, e.g., Li3Sb, Li15Si9, Li15Ge4, Li22Sn5 [5] or by slow reaction kinetics. Presently, great research efforts are focusing on nanostructuring of the active material, by producing nanowires, nanoparticles and nanocrystals (NPs and NCs), in order to mitigate the effects of volumetric changes and to enhance the lithiation kinetics [6]-[13]. With regard to SIBs, it is important to note an even greater need for efficient anode materials, because silicon does not reversibly store Na-ions at ambient conditions [14], graphite shows negligible capacities of 30-35 mAh g−1 [15], while other carbonaceous materials exhibit capacities of less than 300 mAh g−1 at rather low current rates and suffer from the low tap density due to high porosity [12]. Contrary to LIBs, there is much greater progress for the Na-ion cathodes than for Na-ion anodes [16], [17].
In the elemental form, antimony (Sb) has long been considered as a promising anode material for high-energy density LIBs due to high theoretical capacity of 660 mAh g−1 upon full lithiation to Li3Sb [3], [4], and has gained a revived interest as mechanically-milled or chemically synthesized nanocomposites [18]-[21], as well as in form of bulk microcrystalline or thin-film material [22], [23]. Furthermore, stable and reversible electrochemical alloying of bulk Sb with Na has also been recently demonstrated [22], pointing to the utility of this element in SIBs as well. Several reports, published in 2012-2013, have demonstrated efficient Na-ion storage in Sb/C fibers [5], mechanically milled Sb/C nanocomposites [24], Sb/carbon nanotube nanocomposites [25] and in thin films [23].
Monodisperse Sb Nanoparticles (NPs) have been found to show outstanding capacity retention and rate capability as anode material in both LIBs and SIBs [26]. However, the synthesis of monodisperse nanoparticles is tedious, expensive and hardly upscalable. A method disclosed in [26] comprises injecting Sb precursor into a hot solution (150 to 200° C.) containing a mixture of trioctylphosphine, lithiumdiisopropylamide and oleylamide resulting in the formation of Sb(III)oleylamide that is then reductively decomposed to nanoparticles in a size range of 10 nm to 20 nm.
Also already known is the production of antimony nanowires by self-assembling of Sb nanoparticles [27]. The method comprises formation of very small nanoparticles by dissolving the surfactant PVP and SbCl3 in N,N-dimethylformamide and then adding aqueous NaBH4. Aging for several days resulted in nanowires of about 20 nm diameter. Increase of the PVP to SbCl3 ratio from 10:1 to 100:1 resulted in nanowires of 300 nm diameter. Only nanoparticles with an average size of 4 nm are reported.
Another synthesis of Sb nanoparticles is performed by reduction of SbCl3 with lithium triethylboronhydride at room temperature in tetrahydrofurane [28]. This reaction has to be performed under inert conditions, as the employed reducing agent is sensitive to air and moisture. No monodisperse Sb nanoparticles were obtained. Synthesis of mixed antimony nanoparticles starting from such antimony nanoparticles and alloying metal nanoparticles or molecular metal precursor did not result in monodisperse particles either. The particles synthesized by this method were in the range of 20-50 nm [28].
Therefore, the goal was to develop a cheap and facile procedure that allows the gram-scale production of Sb NPs and Sb alloy NPs (SbMx NPs) showing at least the same electrochemical performance as monodisperse NPs.